Try buying more Villages. Their description mentions that they heal you at the start of each day. Also, every 3 Villages increase the healing effect. For example, if you have 2 Villages, each one heals for 2 HP, but once you buy a 3rd Village, each Village will heal for 4 HP instead. Because of that, it’s important to buy as many Villages as possible early in the game so you can keep fighting groups of enemies every day
Also, the Wheat Field card restores 25% of your health when purchased, but I’d say it’s more of an emergency card for situations where you notice that your Villages are no longer healing you back to full HP each day.
Resources work the same way, by the way — the more Sawmills and Mines you buy, the more resources you’ll receive at the start of each day.